Abstract

The design trend for light duty diesel engines is towards lower compression ratio and higher turbocharger boost. This can enable higher specific power and lower pollutant emissions to be achieved, but raises concerns that cold start operation might be adversely affected. This is investigated and quantified through the study of a modern light duty diesel engine at two compression ratios and temperatures down to -20ºC.

Key indicators of cold start performance are the magnitude and cycle-to-cycle variation of indicated mean effective pressure. Initial studies were carried out at 300 rpm, a speed representative of post-first-fire conditions. Studies were then conducted at higher engine speeds representative of cold idle. The utility of different injection strategies, timings and quantities is investigated when varying test temperature and engine speed through a range of values encountered during the cold start phase of engine operation. The importance of the glow plug as a cold start aid is also investigated by varying its operating temperature and protrusion into the combustion chamber. The indicated mean effective pressure was used to assess the effects of varying input parameters, and gross heat release rate information is used to identify the phenomena responsible for desirable or undesirable characteristics.

Reduction in compression ratio led to no deterioration of initial start performance from speeds just above cranking, provided an appropriate injection strategy was chosen. Higher indicated mean effective pressure was possible at low speeds using low compression ratio due to reduced losses and more complete combustion. Cycle-to-cycle variability in indicated mean effective pressure increased markedly for both compression ratios at engine speeds representative of cold idle, especially when test temperature was reduced. Stability reduction was more severe at low compression ratio.

Multiple pilot injections at high compression ratio cold idle resulted in better cycle-to-cycle stability. Analysis of heat release profiles suggested that additional pilots assisted fuel mixing, a conclusion supported by a computational fluid dynamics model. Multiple pilots created a more homogeneous fuel distribution through the bowl at time of main injection. Multiple pilots could not stabilise operation at low compression ratio.

Improvement in cold idle at low compression ratio was achieved by increasing glow plug temperature, which significantly increased the rate of fuel preparation. This increased the initial rate of heat release and resulted in significantly less variation in the heat release rate profiles. Small changes in glow plug protrusion rapidly degraded cold idle performance, indicating the importance of correct design.